The effects of atenolol (2-5 mmol/l), sotalol (1-2 mmol/l) and pamatolol (0.1-1 mmol/l), together with N-tertiary butyl phenoxypropanolamines with o-methyl (D-2T: 50-100 mumol/l) m-methyl (D-3T: 50-100 mumol/l) and p-methyl (D-4T: 100-200 mumol/l) group as well as with o,p-methyl groups (D-24T) (50-100 mumol/l) on action potentials (APs) were investigated in isolated guinea-pig papillary muscles. All the drugs in these concentrations produced a concentration-dependent reduction of the maximum upstroke velocity (Vmax). The reduction of Vmax in premature APs induced by stimuli interpolated between the basic driving rate of 0.25, 0.1 or 0.027 Hz decayed exponentially during diastolic intervals. The time constants of these decay processes tau for atenolol, pamatolol and sotalol ranged between 260-541 ms, those for D-3T and D-4T between 655-1,166 ms, and D-2T and D-24T between 1,565-1,931 ms. A drug which provided larger tau values caused the reduction of Vmax in a wider range of the frequency.(ABSTRACT TRUNCATED AT 250 WORDS)